1. Field of the Invention
This invention relates to electrostatic spray coating systems, and more particularly to the assembly by which spray guns are mounted for spraying coating material onto parts, and to a device for collecting free ions in an electrostatic spray coating system.
2. Description of the Prior Art
In electrostatic spray coating systems, the coating material is pumped from a supply to one or more spray guns which spray the coating material onto a product to be coated. The coating material may be either in the form of dry particles conveyed in a fluidized air stream or in the form of liquid atomized by the gun. The spray guns may charge the coating particles by means of a high voltage charging electrode. When the coating particles are sprayed from the front of the gun, they are electrostatically attracted to the product to be coated which is generally electrically grounded and suspended from an overhead conveyer in a spray booth. The spray guns are mounted in the spray booth, either in a stationary position or on a reciprocator or other device which allows the gun to be automatically moved in a predetermined path. Once these charged coating particles are deposited onto the product, they adhere there by electrostatic attraction until they are conveyed into an oven where they are cured, or, in the case of powder coating, melted to flow together to form a continuous coating on the product.
Previous gun mounts for spray guns have typically comprised a bracket mounted to the gun and connected to a mounting bar installed in the spray booth, with a knob used to tighten the bracket to the bar in the desired position. These gun mounts have performed adequately, but several problems have arisen in the use of these mounts.
The gun slot in the spray booth is sometimes narrow, and when the knob and the pivot from the mount were aligned with the wall of the booth, it could be very difficult for the operator to insert his hand into the booth, grip the knob, and turn the knob to loosen it in order to adjust the position of the gun or to remove the gun from the mounting bar. Even when the gun was in other positions, it could sometimes be difficult for the operator to twist the knob in order to tighten the knob sufficiently so that the gun was held securely.
When these gun mounts have been used on reciprocators, the mounts were subject to loosening, causing the spray gun to rotate and spray downwardly. The center of gravity of the spray gun is in front of the pivot and the knob. Each time the gun reaches the top of the reciprocator stroke, the gun rotates or swings upwardly at the nozzle. When this occurs, the gun swings counterclockwise, causing the gun mount pivot to be urged to rotate counterclockwise as well. This, in turn, caused the knob to rotate counterclockwise, which loosened the knob. Eventually, the knob could be so loose that the gun would pivot freely on the mount and, with the center of gravity in the front of the gun, the gun would swing to a position in which it sprayed downwardly.
The prior art gun mounts have also tended to be difficult to clean. Powder spray guns are subjected to an environment in which powder covers everything in the spray booth. The prior art gun mounts had surfaces that could easily collect powder. The knob, which required rough surfaces in order to be easily operated, was especially subject to undesirable collections of powder.
The prior art gun mounts have also lacked the capability of adjusting the horizontal gun position in order to spray toward the part as it is moving toward the gun on the conveyor, i.e., lead spraying, or to spray the part as it is moving away from the gun on the conveyer, i.e., lag spraying. The capability of lead and lag spraying can be quite useful in certain applications, such as coating vertical corners on the inside of drawers or cabinets. It is desirable that a gun mount allow the gun to be positioned in all possible positions, so that the gun can be moved or placed in the best position for coating a part.
The mounts have also generally been formed of injection molded plastic parts. When guns were moving on a reciprocator, the plastic parts often would be incapable of holding the gun in position so as to minimize gun movement at the nozzle. The two plastic screws which attached the mount to the multiplier would occasionally break. In addition, since the gun mounts were made of a nonconductive plastic material, the gun mounts could not be used for mounting of an anti-back-ionization (ABI) device, since the ABI device would need to be electrically grounded.
ABI devices have been found to be useful in attracting free ions which would otherwise be attracted to the part. Without an ABI device, the free ions are conveyed with the coating material onto the part. This causes a charge to accumulate on the deposited coating layer until the local electric field strength is great enough to cause ionization from the coating layer. This "back-ionization" can disturb the deposited coating and result in craters and other defects in the cured coating. By using an ABI device, the appearance of the surface finish on the part can be improved.
An example of an ABI device is shown in U.S. Pat. No. 4,921,172, issued to Belmain et al., in the form of a counter-electrode mounted on a powder spray gun on the front of the gun. Another example of an ABI device is shown in European Patent Publication No. 0,620,045 in the form of a counter-electrode ring fixedly mounted around the front of the gun. The existing ABI devices have generally been fixed in position or built into the gun and have not provided easy adjustment or removablity.